Even if you flip the main breaker to the off position, the two main leads coming in and attaching to the lugs at the top of the breakers are still hot. Just so you don't think you are completely safe, just be sure where your hands and tools are going that they don't contact the lugs at the top.
Yep, trust me I was VERY cautious on where my hands were during this entire process. Left one in my pocket and wore rubber shoes. Also had the wife in there just in case something happened to me .
Kinda scary I'll admit, but I used common sense and it all went smooth.